An image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor may be generally classified as a charge coupled device (CCD) and a CMOS image sensor.
The CCD may include a plurality of photodiodes (PDs) which are arranged in a matrix, each photodiode converting an optical signal into an electric signal. In the matrix of the photodiodes, a plurality of vertical charge coupled devices (VCCDs) are provided between the respective vertically arranged neighboring photodiodes to transmit electric charges, generated by the respective photodiodes, in a vertical direction, and a plurality of horizontal charge coupled devices (HCCDs) are provided to transmit the electric charges, transmitted by the respective vertical charge coupled devices, in a horizontal direction. Also, a sense amplifier is provided to sense the horizontally transmitted electric charges to output electric signals.
Disadvantages of the CCD include a complicated driving method, high electric power consumption, and a complicated manufacturing process requiring a multi-step photo process. Furthermore, the CCD has another disadvantage in that it is difficult to integrate a control circuit, a signal processing circuit, an analogue/digital conversion circuit (A/D converter), etc. in a CCD chip. This makes it impossible to achieve compact-sized products.
Recently, the CMOS image sensor has received much recognition as a next-generation image sensor to solve the problems of the CCD. The CMOS image sensor is a device employing a CMOS technology. The CMOS image sensor uses a control circuit, a signal processing circuit, etc. as peripheral circuits, such that MOS transistors, having the same number as unit pixels, are formed on a semiconductor substrate. The outputs of the respective unit pixels are sequentially detected by the MOS transistors by a switching method. Meaning, the CMOS image sensor is characterized in that a photodiode and a MOS transistor are formed in a unit pixel to sequentially detect electric signals of the respective unit pixels by a switching method, thereby achieving an image.
As a result of employing a CMOS fabrication technology, the CMOS image sensor has several advantages, for example, low electric power consumption, and a simplified fabricating process based on a reduced number of photo process steps.
Example FIGS. 1A to 1C illustrate a method for fabricating a CMOS image sensor and which may include dielectric film 11, such as a gate dielectric film or an interlayer dielectric film, formed on and/or over semiconductor substrate 10. Metal pad 12 for each signal line is formed on and/or over dielectric film 11. Passivation film 13, made of an oxide film or a nitride film, is formed on and/or over the entire surface of dielectric film 11, including metal pad 12.
As illustrated in example FIG. 1B, photosensitive film 14 is applied to the top of passivation film 13 and patterned by an exposure and development process to expose the top of metal pad 12. Passivation film 13 is selectively etched using the patterned photosensitive film 14 as a mask to form opening 15 at the exposed metal pad 12.
As illustrated in example FIG. 1C, a silicon nitride film or a silicon oxide nitride film is deposited on and/or over the entire surface of semiconductor substrate 10 to form planarization layer 16. Planarization layer 16 is selectively etched by a photo and etching process such that planarization layer 16 is left only at the remaining area excluding the metal pad. Color filter layers 17 are formed on and/or over planarization layer 16 corresponding to respective photodiode regions. The respective color filter layers are formed by applying respective color resists and performing a photolithography using an additional mask. Subsequently, overcoat layer 18 is formed on and/or over the entire surface of semiconductor substrate 10, including color filter layers 17. The remaining area, excluding metal pad 12, is selectively etched by a photo and etching process. A polymer material is then applied to the top of overcoat layer 18 to form a micro lens material layer. Photosensitive film 14 is patterned by an exposure and development process to define a micro lens region. Subsequently, the micro lens material layer is selectively patterned using photosensitive film 14 to form a micro lens pattern corresponding to color filter layers 17. The micro lens pattern is heat-treated by a reflow process to form hemispherical micro lenses 19 having a predetermined radius of curvature.
In the afore-described method for fabricating the CMOS image sensor, however, color filter layers 17 and planarization layer 16 form a sharp step between a scribe lane and a pixel region, before the formation of the micro lenses. Accordingly, the film thickness of the applied photo resist is non-uniform. The non-uniform thickness of the photo resist, in turn, causes non-uniformity in the thickness of micro lenses 19. For a spin coat type resist application, such non-uniformity of thickness causes striation in which radial stripes are formed outward from the center of a wafer. The striation causes defectiveness in which vivid stripes are diagonally formed on the CMOS image sensor.
Moreover, in the afore-described method for fabricating the CMOS image sensor, the metal pad is opened for connection with an external circuit, and then the process for forming the color filter layers and the micro lenses are carried out. All the processes are photolithography processes using a strong base developing solution, which may result in corrosion of the grounded metal pad.